1. Field of the Invention
The present invention relates to meter mechanisms for electromechanical instruments, and more specifically to a flight instrument having an indicator which is positioned by a miniature servomotor and a novel capacitive position feedback transducer.
2. Description of the Prior Art
Flight instruments having a meter movement for positioning an indicator or pointer are well known in the art and a representative example is described in U.S. Pat. No. 3,577,195, by C. Clift, issued May 4, 1971, and assigned to the applicants' assignee. The aforementioned Clift patent discloses a D'Arsonval type of meter movement which is responsive to a control signal and which has an auxiliary a.c. excitation winding on the field magnet, thereby providing both a unidirectional flux field and an alternating flux field for the meter movement magnetic circuit. The meter armature has two windings. The first winding is excited by the control signal and reacts with the directional magnetic field to displace an armature, and the second winding operates to sense both the displacement of the armature and the rate of change of the displacement by acting respectively with the alternating and unidirectional magnetic fields. When included in a closed loop servo system the meter movement operates as both a servodrive and an angular motion transducer for providing displacement and rate feedback terms.
Flight instruments, for example an attitude director indicator, typically include pointers and arms coupled to the armatures of the above described type of meter movement. The travel arcs of these pointers and arms have their centers of curvature or pivot points disposed at some distance from a display surface. This arrangement results in a major alignment problem for the pointers since small displacements of the meter movement bodies, which are subject to mounting constraints, effect the pivot points and are amplified through the arms to the pointers.
The prior art has attempted to solve these alignment problems with relatively complex devices of the type described in U.S. Pat. No. 3,599,597, by Maday et al., issued Aug. 17, 1971, and assigned to the applicants' assignee. The apparatus of the Maday patent provides means for aligning the meter pointers so that they have adequate clearance from other components. For example, in an attitude flight indicator, each meter is disposed relative to three set screws arranged in a triangular pattern and threaded through a meter support structure into contact with the meter. To orient the pointer, the meter is temporarily loosened from its rigid connection to the support structure. This enables the set screws to be adjusted so as to move the meter pointer relative to an indicator housing. After the pointer has been appropriately positioned so that it can move along a prescribed path without contacting other components, the meter is again rigidly clamped to the support structure in readiness for normal operation. Additional set screws adjusted in relation to other meters permit the individual pointers to be separately aligned in a similar manner. It can be appreciated from the foregoing that flight instruments utilizing the apparatus of the Maday patent are relatively difficult and time consuming to manufacture.
Accordingly, there is a need for a servo controlled pointer system suitable for use in flight instruments and other electromechanical instruments which alleviates the alignment and manufacturing problems associated with prior art systems.